Field of the Invention
The present invention relates to surface coated steel materials in various forms having a manganese coating thereon and a fine and compact hydrated manganese oxide formed on the manganese coating, which steel materials show excellent corrosion resistance, workability and weldability.
As well known, the means for providing corrosion resistance for a steel material includes:
(1) Addition of alloying element (for example, stainless steels, atmospheric corrosion resistant steels, etc.) PA1 (2) Organic coatings and inorganic coatings (for example, paints, synthetic resins, mortar, enamels, etc.) PA1 (3) Metallic coatings (for example, zinc, tin and aluminum coatings, etc.) PA1 (1) Brittleness PA1 (2) Chemical reactivity (a short service life in an aqueous solution or outdoors) PA1 (3) Dark color of corrosion products (unsuitable for ornamental purposes yet suitable for a protective coating).
Among the above surface protective means, the metallic coatings have been most widely used, and zinc-coated steel materials, in particular, have been and are used in tremendous amounts for manufacturing materials for buildings, automobiles, electric appliances and also used in the forms of wires and sections.
However, as zinc-coated steel materials have been increasingly used in various applications as mentioned above and under severe service conditions, a conventional single zinc-coating or single metal coating has not always been able to satisfy requirements and recent trends are that a composite or alloy coating is applied to steel materials so as to improve the properties.
This is due to discoveries and knowledges obtained through long-year experiences that the corrosion resistance effect of zinc (or zinc alloy) based on its nature that it is electrochemically baser than iron, namely due to its sacrificial anodic action, can not be maintained if the corrosive media is very severe and the dissolution of zinc is so rapid.
For example, referring to a colored galvanized iron, which has been widely used for building materials, a zinc-coated or alloyed zinc-coated steel plate is used.
However, the environments to which the zinc-coated or alloyed zinc-coated steel sheet is exposed usually contain corrosive media, such as water, oxygen and salts, so that the coated zinc dissolves in a very short period of service, thus developing red rust due to the corrosion of the base steel sheet, and further promoting the corrosion of the base steel sheet itself. Therefore, the zinc-coated steel sheet is seldom used in this field without a further surface treatment.
Thus, the zinc-coated steel material is usually subjected to a surface conversion treatment, such as chromating and phosphating, suitable for zinc, after the zinc coating, and further subjected to organic coatings compatible to the surface conversion treatment for the purpose of improving the corrosion resistance and in view of the ornamental appearance. However, even when a steel material is coated with a composite coating of the zinc-coating, the conversion coating and the organic coating, the coated zinc is first attacked easily by the corrosive substance, such as water, oxygen and salts which permeate through the organic coating, and then the organic coating itself is apt to be easily destroyed by the substances produced by the corrosion of the zinc coating. Further, in the case where the conversion treatment, such as chromating is done for the purpose of improving the adhesion with an organic coating, there is a problem of public pollution due to the hexavalent chromium ion present in the chromate film. Therefore, strong demands have been made for development of a surface treated steel sheet having improved corrosion resistance than the conventional materials.
As mentioned above, in the case when a zinc-coated steel material having an organic coating on the zinc coating, the corrosion resistance of the zinc coating itself is very important, just as when the zinc-coated steel material is used without an organic coating thereon, and for this reason the recent technical tendency is directed toward inhibition of the sacrificial anodic action of the coated zinc and commercial trials have been made to artificially make the galvanic electrode potential of the zinc coating approach to that of iron by alloying the zinc coating with iron, aluminum, nickel, molybdenum, cobalt etc. resulting in developments of Zn-Fe alloy coated, Zn-Al alloy coated, Zn-Mo-Co alloy coated steel products, which are now in the market.
These alloyed zinc coatings are said to have a corrosion resistance 2 or several times better than that of the conventional zinc coating, but the Zn-Fe alloy coating has difficulty in working, the Zn-Al alloy coating has problems in workability, weldability and paintability, thus failing to provide a coated material having a satisfactory integrated property, and although the Zn-Mo-Co alloy coating seems to provide the desired integrated property, it is very difficult to form the alloy coating of uniform composition, because each of the component metals shows a different electrodeposition speed depending on the electroplating conditions.
Therefore, in recent years strong demands have been made in various fields for the balanced property, namely for a commercial development of a surface coated steel material having excellent workability and weldability as well as satisfactory paintability and adaptability to chemical conversion treatments, but up to now, there is no surface coated steel material which can meet with the above requirements.
For improving the corrosion resistance of a steel material by coating the steel material with other metals and utilizing the corrosion resistance of the coated metals, there are two groups of coating methods, as classified electrochemically; the first group in which a metal nobler than iron is coated, for example chromium plating; the second group in which a metal baser than iron is coated, for example, zinc plating. For the first group of methods, many studies have been made and many arts have been established. However, when the metal coating itself has pinholes, or when the thickness of a coating increases, the coating is susceptible to cracking, as seen in the chromium coating. In either case, the metal coating has a defective portion, so that the steel substrate is first attacked because iron is electrochemically baser than the coated metal, just contrary as in the zinc coating, so that pitting corrosion is apt to occur, thus deteriorating the reliability of the coated steel material.
In view of the above facts, it may be concluded that a metal, such as zinc, which shows the sacrificial anodic action is more advantageous for protecting steel materials from corrosion. The present inventors made systematic studies in consideration of the above technical points of view, and have found that among various coated steel materials, a manganese coated steel material having a hydrated manganese oxide formed thereon shows the best corrosion resistance. As clearly understood from the galvanic series of metals in an aqueous solution, as manganese is electrochemically baser than zinc, it has been undoubtedly expected that manganese has an inferior corrosion resistance as compared with zinc.
Regarding the electrodeposition of manganese, many various studies have been made including "Electrolytic Manganese and Its Alloys" by R. S. Dean, published by the Ronald Press Co., 1952; "Modern Electroplating" by Allen G. Gray, published by John Willey & Sons Inc., 1953; "Electrodeposited Metals Chap. II, Manganese" by W. H. Safranek, published by American Elsevier Pub. Co., 1974, and "Electrodeposition of Alloys", Vol. 2 "Electrodeposition of Manganese Alloys" by A. Brenner, published by Academic Press, 1963.
According to R. S. Dean, the electrodeposition of manganese and its alloys act self-sacrifically anodically just as zinc and cadmium in the aspect of rust prevention, and a steel sheet having 12.5.mu. thick manganese coating can well resist to the atmospheric exposure for 2 years, and Allen G. Gray reported by citing "Sheet Metal Industry", 29, p. 1007 (1952) that a satisfactory protective effect can be obtained by a thick manganese coating and that the electrolytic manganese becomes black when exposed to air, but this can be prevented by an immersion treatment in a chromate solution.
Further, according to N. G. Gofman, as reported in "Electrokhim Margantsa" 4, pp. 125-141 (1969), the electrodeposited manganese corrodes in the sea water at a rate by 20 times faster than zinc, but the corrosion rate of manganese can be decreased when a chromate film is provided on the manganese.
What is more interesting is reported by A. Brenner. He pointed out the following three defects of the manganese or its alloy coatings, although he mentioned a protective film for steels or low alloyed steels as one of the expected applications of the manganese or manganese alloy coatings.
Regarding the brittleness, manganese electrodeposited from an ordinary plating bath, has a crystal structure of .gamma. or .alpha., and the .gamma. structure which is softer transforms into the .alpha. structure when left in air for several days to several weeks. Therefore, in practice, considerations must be given to the .alpha.-manganese. In this case, the hardness and brittleness are said to be similar to those of chromium, i.e. 430 to 1120 kg/mm.sup.2 expressed in microhardness according to W. H. Safranek.
Regarding the chemical reactivity, A. Brenner reported that the manganese or its alloys can be stabilized by a passivation treatment in a chromate solution, and the thus stabilized manganese or its alloys can stand satisfactorily stable for a long period of time in the indoor atmosphere, but he pointed out that for outdoor applications an eutectoid with a metal nobler than manganese should be used.
Therefore, judging from the fact that a zinc coated steel sheet with zinc coating of 500 g/m.sup.2 by hot dipping can protect the steel sheet against corrosion for 30 to 40 years, a zinc coating of 90 g/m.sup.2 by hot dipping which corresponds to a manganese coating of 12.5.mu. can be predicted to resist the atmospheric corrosion at least for 5 to 6 years, therefore a manganese coating which can resist to the atmospheric corrosion for only 2 years can not be said to have a better corrosion resistance than a conventional surface treated steel sheet.
Up to now no trial or study has ever been made to improve the corrosion resistance of a steel material by manganese coating thereon, except for the invention made by the present inventors as disclosed in Japanese Patent Laid-Open Specifications Sho 50-136243 and Sho 51-75975.
The present invention is clearly distinctive over these prior arts in the following points.
The Japanese Patent Laid-Open Specification Sho 50-136243 discloses a surface treated steel substrate for organic coatings, which is obtained by electro-plating 0.2.mu. to 7.mu. manganese coating on the steel material, and by subjecting the manganese coated steel material to a chromate treatment or a cathodic electro-conversion treatment in a bath of aluminum biphosphate or magnesium biphosphate or both. The technical object of this prior art is to facilitate the conversion treatments by coating manganese because it is difficult to apply in substitution for zinc coating conversion treatments such as the chromate treatment and aluminum biphosphate and magnesium biphosphate treatments directly to the steel material, and also it has an object to improve the paintability and further the corrosion resistance.
The Japanese Patent Laid-Open Specification Sho 51-75975 discloses a corrosion resistant coated steel sheet for automobile, which comprising a steel substrate containing 0.2 to 10% chromium and at least one layer of coating of zinc, cadmium, manganese or their alloys in a total thickness of 0.02.mu. to 2.0.mu.. This prior art is based on the fact that when the chromium content exceeds 0.5%, the crystal formation on the surface becomes increasingly scattered during the phosphate treatment, for example, and when 3% or more of chromium is contained, completely no phosphate crystal is formed, so that an excellent corrosion resistance of a steel substrate can be obtained, and that it is effective to apply only on the steel surface a single layer or multiple layers of coating of zinc, cadmium, manganese or their alloys which are very reactive to the conversion treatments.
As explained above, the prior arts which were also made by the present inventors utilized the nature of manganese that it has a stronger chemical reactivity than zinc for improvement of applicability of a steel material to chemical conversion treatments, and provide a steel substrate for paint coating. Therefore, these prior arts did not review the corrosion resistance of the hydrated manganese oxide formed on the manganese coating.
The reason why the manganese coating exhibits excellent corrosion resistance is that the thin layer of the hydrated manganese oxide formed on the metallic manganese coating is hardly dissolved in water, and serves as a kind of passivated film and contributes to corrosion resistance as contrary to a pure manganese metal which is very reactive.
Thus when metallic manganese is electrochemically deposited using a usual sulfate bath, the metal manganese reacts with oxygen in the air, and manganese hydroxide formed in a thin film during the electroplating is oxidized by the air and the oxygen-containing manganese compound is formed according to the following formulae (1) and (2). EQU 2Mn(OH).sub.2 +O.sub.2 .revreaction.2H.sub.2 MnO.sub.3 ( 1) EQU H.sub.2 MnO.sub.3 +Mn(OH).sub.2 .revreaction.Mn.MnO.sub.3 +2H.sub.2 O (2)
This oxygen-containing manganese compound hardly dissolves in a neutral salt solution or in water and provides a very stable corrosion resistant film, completely different from the metallic manganese.
An oxygen-containing metal compound, such as the oxygen-containing manganese compound, is known to contribute to corrosion resistance just as a stainless steel exhibits excellent corrosion resistance due to its passivated surface film of a hydrated oxide containing 20 to 30% water, and a thinly chromium coated tin-free steel exhibits excellent corrosion resistance and excellent paintability due to its oxyhydrated chromium compound film containing about 20% water. It is also known that the rust of steel exposed to the air for a long period of time contains non-crystalline oxyhydrated iron compound, FeOOH, and that the rust layer of an atmospheric corrosion resistant steel which exhibits excellent resistance to atmospheric corrosion contains much of such oxyhydrated iron compound.
As described above, in the case of manganese, too, the oxygen compound containing water in the film is considered to have a remarkable effect on the corrosion resistance, and particularly advantageous in the corrosive environments, such as the marine splash zone, where Cl.sup.- ion is a main corrosion factor and highways where salts are sprayed for the purpose of prevention of freezing as practised in U.S.A., Canada and Europe, because Cl.sup.- ion tends to promote the transformation of Mn.MnO.sub.3 to MnOOH having better corrosion resistance.
The prior arts as disclosed in Japanese Patent Laid-Open Specifications Sho 50-136243 and Sho 51-75975 took no consideration to the hydrated manganese oxide formed on the manganese coating, or regarded it as a corrosion product which damages the ornamental value. The present invention, for the first time, intends to form intentionally this hydrated manganese oxide on the manganese coating and utilize it advantageously.
Detailed descriptions will be made on corrosion of steels in marine environments.
Steel materials have been also widely used in marine structures because they cost low and easy to work. However, the marine environment is quite different from ordinary environments and is very severely corrosive to the steel materials due to the salt, and special considerations against the sea water corrosion must be taken.
The corrosion of a large steel structure extending continuously from the sea bottom upward above the sea surface is schematically shown in FIG. 8, from which it is understood the most severe corrosion is seen in the "splash zone" and the portion just below the ebb tide line.
The reasons why the corrosion is severe in the splash zone are considered as that the sea water is intermittently splashed over the structure and the steel is heated by the sun to a considerably high temperature, so that the steel is exposed to alternative repetition of drying and wetting under a heated condition and the corrosion is promoted so rapidly that the corrosion rate per year can reach 0.3 to 0.5 mm in average.
Meanwhile, the reasons for the severe corrosion of the steel in the portion just below the ebb tide line are considered as that the portion above the ebb tide portion is supplied with more oxygen than the portion below the sea surface, so that a so-called galvanic cell is formed between the portion just below the sea surface and the portion just above the sea surface and the portion just below the sea surface is more attacked while the portion above the sea surface is less attacked, the former corrosion rate reaching as much as 0.1 to 0.3 mm per year as compared with 0.1 mm or less per year of the latter corrosion.
The corrosion of the steel material, somewhat deeper in the sea, is 0.05 to 0.1 mm per year, depending on factors such as the oxygen dissolved in the sea water, the sea water temperature, the velocity of the sea water, the quality of the sea water, and the bacteria in the sea water, etc.
Meanwhile, the corrosion of the steel materials in the sea bed is much less, because the dispersion of the dissolved oxygen is the slowest.
As described above, the corrosion of steel materials in the marine environments varies depending on the positions at which the steel materials are used, and a preventive means against the corrosion of the splash zone has been regarded as the most important in the marine applications.